Abstract

Simulations of high-resolution 19F-decoupled 27Al and 23Na magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra of the aluminofluoride minerals, cryolite, cryolithionite, thomsenolite, weberite, chiolite, prosopite, and ralstonite combined with theoretical modeling have given accurate values of chemical shift (δiso), and quadrupolar interaction parameters (Cq and η), thereby eliminating ambiguities incurred by the complex nuclear interactions. These NMR data have been correlated with local electronic environments in the minerals, which were calculated using Full Potential Linearized Augmented Plane Wave (FP LAPW) modeling based on the structures from X-ray diffraction (XRD) data. This combination of NMR, XRD, and modeling techniques allowed the analysis and optimization of the crystal structures.

The electronegativities and distances of neighboring ions, represented here by an environmental parameter χ, are shown to control δiso of both 23Na and 27Al. The calculations using χ, also show that the ions beyond the nearest neighbor play an important role in determining δiso of 27Al and 23Na in these aluminofluoride minerals, and the substitution of OH for F significantly affects the shielding around 27Al in prosopite and ralstonite. There is a positive correlation between the site distortion at the Na and Al sites and the values of Cq in these aluminofluoride minerals.